In wireless communications systems, copies of a transmitted signal arriving at a receiver can cause problems with the receiver's ability to accurately decode the transmitted signal. The multiple copies of the transmitted signal arriving at the receiver may be caused by reflections of the transmitted signal arriving at the receiver at different times due to different distances traveled by the different reflections. This phenomenon is commonly referred to as multipath.
In a receiver of a spread spectrum communications system, the individual paths in the multipath can often be combined to help improve the signal quality of the received signal and therefore improve the probability of accurately decoding the information received. This can be accomplished by the use of a rake receiver, which has a plurality of fingers, wherein each one of the fingers can be assigned to track an individual path of the multipath, to demodulate a version of the received signal (the received signal from each finger may be slightly different due to distortion induced by reflections) from the assigned path in the multipath. The various versions of the received signal demodulated by the fingers may then be combined to produce a single received signal that may be of better quality than any single received signal from any one of the individual paths.
However, since in many applications, receivers are placed in small wireless devices, resources (such as the number of fingers in a rake receiver, overall processing power, and so forth) can be limited due to size, power, and cost constraints. Additionally, improper assignment of fingers to paths can result in multiple fingers being assigned to the same path. This may arise from the assignment of fingers to paths that are too close to one another, and then, through timing adjustments, one (or more) of the fingers moving closer and closer to a single path until all involved fingers are demodulating the same path.
Furthermore, the operating environment for a wireless device can rapidly change due to movement of the wireless device. The changes in the operating environment can change the number and the position of the individual paths in the multipath. The receiver needs to be able to detect the changes in the paths and make changes to the finger assignments in a timely manner. Therefore, the management of the finger resources (i.e., assigning fingers to paths, determining which paths to demodulate and which paths to ignore, and so on) can be a vital factor in the overall performance of the receiver.
One prior art technique continuously monitors the strength of the path and allocates fingers to a path if its strength is greater than a specified threshold. Another prior art technique compares the strength of various paths and can either allocate a finger to the strongest path or change an assignment for a finger from an existing path to the strongest path. Yet another prior art technique maintains a time separation between fingers by not permitting a timing adjustment that can result in a spacing between fingers that is less than permitted.
One disadvantage of the prior art is that the continuous monitoring of the paths and their strengths may require a large bank of correlators or a matched filter to perform the monitoring. The continuous operation of the large bank of correlators or a matched filter may consume a large amount of power, which can reduce the battery life of a wireless device.
A second disadvantage of the prior art is that the comparison of the strengths of the various paths and the allocation of a finger to demodulate the strongest path does not perform well in the presence of a fading channel.